Achieve a low Langelier index and mantain water pH between 7,2-7,6

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Hi,
I think our problem should be very common for enterprises which get water by desalinisation and have to follow our laws:
The enterprise I work for get all the water by desalinisation, (400m3 daily). It have to be conducted to a head deposit, to a regulation tank and from there on into the drinking water net. Most of the water is for the pools of a waterpark,(150m3), and plant irrigation, (200m3). Only 50m3 are for drinking water.
Spanish Authorities want us to keep the Langelier index at +-0,5. As the origin is desalinisation, the water is very corrosive: -2,5 is the current Langelier index with a pH of 7,2. To regulate it, we use calcium carbonate. It wouldn`t be so difficult if we could mantain the pH around 8. But as most of the use is for a waterpark, desirable pH would be 7,2-7,6 and for plant irrigation, pH 7. It would be very expensive to add calcium carbonate firstly and acidify most of the volume again at the end of the drinking water pipe. Any building solution was refused for the next years, (separate the supply).
Do you know any chemical solution to mantain the Langelier index low and keep the pH also as low as possible? I understand this is quite difficult, as there is direct correlation between pH and Langelier index.
I would be very glad about any tip, even if somebody could say me: this is impossible.

Regards,
Us, (Agric/Health safety)

 

[bimr]

Calcium carbonate is not typically used for this application, unless you are talking about the use of lime (which is calcium hydrozide, not carbonate).

Post Treatment (Hydrated Lime & Carbon Dioxide Addition; PostChlorination) should be used to re-mineralise the product water, which will then enter the water distribution system.

Lime will increase calcium hardness and raise ph. Addition of carbon dioxide to increase alkalinity (carbon dioxide converted to bicarbonate after hydrated lime addition)

Chlorine should be added in the final stage for disinfection.

What you are saying is impossible. If you want a positive langelier, you need a high pH. You are not going to get a positive langerlier with a low pH.